1. Area of the Invention
This invention concerns a novel design of a device for the extraction of wave energy by means of which the mechanical energy contained in waves may be transformed into a usable form of energy.
More specifically, this invention relates to a device by which wave energy can be extracted by utilising the oscillating horizontal component of the wave motion at a series of points along a stationary, floating structure. Due to the special design of the device, the construction and initial costs will be low, hence it will be possible to extract the wave energy in an unprecedented cost-effective manner.
As a result of the low initial costs, the type of device mentioned here may even be used in near-shore areas and internal waters where the form and size of the waves would normally not allow for installation of commercially sustainable wave power generators.
An additional application of the device mentioned here would be as coast protection. As the device drains the kinetic energy from the waves, installations of this type of device will have a dampening effect on the waves when they reach the shore.
Another advantage of the device mentioned here is that it has little or no visible exposure in the environment in which it operates. The reason for this is that the device operates fully or partially submerged hence keeping its visual profile low.
Another advantage of the device mentioned here is that it comprises of small units that work independently, thus it is not functionally critical if some of these units become defective. The plant will still work—only with a slightly reduced effect even though there are defective units. These defective units may thus be repaired or replaced one at a time when this is timely.
2. Description of Prior Art
Over the years, numerous devices and methods have been suggested for the extraction of the kinetic or potential energy from waves and for the transformation of this energy into a usable form, either as mechanical energy, electrical energy or heat.
When watching a wave on a surface, it is easy to get the misconception that waves are columnar bodies of water oscillating at the vertical level without the existence of any horizontal motions.
If the wave motion is projected on a vertical level perpendicular to the propagation direction of the wave, it is, however, recognised that a wave motion is not a one-dimensional, but a two-dimensional motion. This may e.g. be recognised by observing the motion pattern of an element floating on a wavy water surface. If the motion of the floating element is recorded, it will become evident that it describes a circulating motion, perpendicular to the water surface and parallel to the propagation direction of the wave.
If the kinetic energy is measured from a wave at the vertical level perpendicular to the propagation direction of the wave, it is revealed that the mean kinetic energy in this level is equivalent to the potential energy of the waves as a result of the difference between the trough and crest of a wave.
Thus, in a wave motion there is a constant shift between kinetic and potential energy, very analogous to the shift between kinetic and potential energy that exists in the oscillating motion of a pendulum.
Literature describes a wide range of different wave power plant designs.
In literature, the most frequent plant type is equipped with tethered floats utilising the difference of height between the crest and trough of the waves. As an example of this plant type, U.S. Pat. No. 870,706 can be mentioned in which a number of floats move up and down, driven by the variations of the water surface level.
A mode of operation equivalent to the one mentioned above is applied in the European patent application EP 1045138A2 where the wave energy is used to pressurise a pipe from which a flow of pressurised water can be drawn off.
A method for direct extraction of electrical energy with a point-shaped buoy is presented in U.S. Pat. No. 5,347,186. This patent concerns the generation of electrical energy by means of a linear generator containing a movable magnet. If the generator is accelerated along the magnet's direction of motion, the magnet mentioned above will shift internally in the generator, which can be used to generate electrical energy. If one or more of this type of generators are placed on a buoy, these generators will continuously generate electrical energy qua the constant motion of the buoy. Even though this type of generators can easily be installed on existing buoys and function as power supply for these, this type of generators is not suitable for generating an electrical effect, which can recover the installation costs of the buoy.
A significant problem for all of the plants mentioned above is that their net generated power is low in relation to the initial costs. The low efficiency is due to the fact that these generators can only extract energy for an area corresponding to the area of the float. As the applied float is much smaller than a typical wavelength, there is thus a distinct limitation on the maximum theoretical output per float as the float only affects a small part of the wave area.
Even though wave power systems with vertically movable floats are intuitively easier to understand, literature also describes systems where the horizontal and not the vertical component of the wave's energy is utilised.
One of the earliest examples of this type of plant is found in U.S. Pat. No. 875,950 where a number of submerged resistance elements are forced back and forth as a result of the horizontal component in the circulating wave motion. The resistance elements according to U.S. Pat. No. 875,950 do not oscillate freely, as the motion is dampened by one or more pump units which force seawater under pressure into a collector pipe due to the forced motion of the resistance elements. This collector pipe is lead to a turbine station where the pressurised seawater is used to power a turbine.
A problem in connection with this principle described in U.S. Pat. No. 875,950 is that the resistance elements are hung up as vertically hinged doors. In practice, this means that only a small part of the resistance elements move with optimal speed in relation to the motive wave as the part close to the centre of rotation will move too slowly in relation to the horizontal motion of the wave, whereas the part of the resistance elements, which is farthest away from the hinge point, will move too fast in relation to the horizontal motion of the wave.
Another condition, which makes the device described in U.S. Pat. No. 875,950 unusable in practice, is that the horizontal component of the wave motion decreases with depth. Thus, the horizontal component of the wave motion will be all but extinct near the bottom where the resistance elements are located.
A similar, but more effective method for the application of the horizontal component of the waves is described in WO application 9817911. Unlike the procedure described in U.S. Pat. No. 875,950, the resistance element is in this case anchored with hinges at the bottom of the sea, which implies that there is not much inconsistency between the motion of the wave and the motion of the resistance element. Furthermore, this invention has the advantage that it does not contain significant static elements which should stay put in relation to the relative motion of the water, this will reduce the stress of the water on the plant.
A common problem for the methods mentioned above for the utilisation of wave energy is that the initial costs are excessive relative to the installed power. This condition is due to the fact that existing methods for the utilisation of wave power are based on locally anchored point-shaped resistance elements. These local anchoring points must be severely over-dimensioned as they must be able to handle the force load on the structure during a storm.
As the construction and anchoring costs constitute a big part of the total initial costs, there is thus an approximate proportionality between the energy, which the system may potentially generate, and the initial costs for building the system.